Method for the extraction of alkali metals from their amalgams



June 12, 1956 METHOD FOR THE EXTRACTION OF ALKALI METALS VANHAREN ET ALFROM THEIR AMALGAMS Filed April 1, 1952 I)? van for! L amb en Van 17 wPierre 0c fans United States Paten 0,...

METHOD FOR THE EXTRACTION OF ALKALI METALS FROM THEIR AMALGAMS LambertVanharen and Pierre Octors, Brussels, Belgium, assignors to Solvay &Cie, Brussels, Belgium, a Belgian company Application April 1, 1952,Serial No. 279,802

Claims priority, application Netherlands April 4, 1951 11 Claims. (Cl.7566) The present invention relates to a new process for the extractionof alkali metals from their amalgams, particularly from amalgamsproduced by the electrolysis of aqueous solutions of alkali metal saltsin a cell provided with a mercury cathode.

It is known that these metals, especially sodium which has acquired avery great importance commercially, are produced on a factory scalealmost exclusively by the electrolysis of fused salts.

This process has, however, serious drawbacks, because the. reaction mustbe eifected at a very high temperature, for insatnce above 800 C. whenelectrolysing fused sodium chloride, and above 300 C. when electrolysingfused caustic soda. The power consumption (approx. 15

kWh/kg. Na) and the deterioration of the apparatusbecause of corrosionand of high temperatures, make this process very troublesome.

Various processes have already been proposed in order to replace thethermal electrolysis by the extraction of the alkali metal from theamalgam obtained economically by the electrolysis of an aqueous solutionof an alkali metal salt in a cell provided with a mercury cathode.

The difiiculties inherent in these processes result from the weakconcentration of the alkali metal from the amalgams obtained in thefirst phase of the process.

It has been particularly recommended to extract the alkali metal from anamalgam by the distillation of mercury. This working method istheoretically very economical, because, by judiciously combining theproduction of an alkali metal with an electric power generator based onthe recuperation of the heat of condensation and of the sensible heat ofthe evaporated mercury, it is possible to obtain this alkali metal witha minimum consumption of energy. However, this process has not found anyindustrial application because of the insurmountable difficultiesencountered in the realisation of the apparatus. In fact, thedistillation of mercury from the amalgam in order to achieve a completeseparation of the sodium entails working conditions, and particularlythe application of temperatures of about 900 C. under atmosphericpressure to which the materials used in the construction of theindustrial apparatus are subjected, cannot resist for long.

The alkali metal obtained according to this process has also thedisadvantage of containing comparatively large quantities of mercurywhich must be separated, for instance by a combination with an alkalineearth metal, the afiinity of which for mercury is superior to that ofthe alkali metal.

The quantity of mercury to be separated from the amalgam by distillationmay obviously be reduced by the prior concentration of the diluteamalgam obtained by electrolysis: the dilute amalgam is cooled so as toprecipitate a more concentrated amalgam which is separated from themercury that remained liquid. This improvement does not, however, permitof an avoidance of the' critical phase of the distillation at a hightemperature, so that the process remains practically impossible ofrealisation.

ICC

The present invention aims at avoiding the inconveniences of the abovedescribed processes and particularly at obtaining a technically purealkali metal, rigorously freed from mercury, starting from amalgamsobtained by the electrolysis of aqueous solutions of alkali metal saltsin a cell provided with a mercury cathode.

The present invention is based on the fact that the alkali metal of anamalgam previously concentrated may be extracted by a solvent,particularly by liquid ammonia or by a fused alkali hydroxide.

We are aware of the fact that it has already been proposed to decomposethe amalgam .in the presence of a solvent of the alkali metal in anelectrolytic cell in which the amalgam is the anode, but this process isessentially different and requires two consecutive electrolyses,starting at an elevated power consumption which may be estimated in thecase of sodium at about 10 kwh./ kg. of alkali metal.

According to the present invention, the dilute amalgam obtained by theelectrolysis of an aqueous solution of an alkali metal salt in a cellprovided with a mercury cathode is concentrated in such a manner as toachieve a concentration of alkali metal of more than 10% by weight,which may be brought about by working at temperatures not prejudicial tothe apparatus, then the concentrated amalgam is treated with a solventin order to dissolve an important part of the alkali metal contained inthe amalgam. The exhausted amalgam is recycled towards the apparatus forconcentration, whilst the alkali metal is separated from the solvent.'

The concentration of the alkali metal of the amalgam prepared by theelectrolysis of an aqueous solution of an alkali metal salt does notusually exceed 0.7% by weight.

At higher concentrations, the amalgam begins to solidify at thetemperature usually maintained in the elec trolytic cells. This diluteamalgam may be concentrated by distillation at elevated temperatures,preferably in several successive columns, so as to carry theconcentration of the alkaline amalgam to such a value as to rendereconomical the extraction with a solvent. The lower concentration limitobviously varies with the nature of the sol-' vent, but the applicantshave found that, in the case of the known solvents, the higherconcentration limit is at 10% by weight.

Before the distillation, it is obviously possible to eifect a priorconcentration bycooling the hot dilute amalgam of the cells in such amanner as to precipitate the solid amalgam. In the case of an amalgam ofsodium, for instance, it is possible to precipitate NaHg4 (about 2.8% ofNa) which is separated from the liquid mercury. The concentration of theamalgam is then raised by distillation, if necessary under reducedpressure.

According to a preferred method of working the process, theconcentration of amalgam which is to be submitted to extraction by asolvent is brought to the maximum value compatible with an economiccourse of the distillation of mercury; for example, in the case of theproduction of metallic sodium, the amalgam is preferably brought to aconcentration in Na of from 40 to 60%.

At these concentrations, the solubility of Na in the solvent is in factelevated and the distillation temperature of mercury contained in theamalgam remains within limits consistent with the materials ofconstruction of the apparatus to attack by sodium and mercury. Theresult is that to a comparatively slight increase in the amount of evap,orated Hg corresponds a slight recycling of amalgam and Patented June12, 1956 to alkali metals may be suitable in so far as their dissolvingpower is sutficient to allow the economic extraction of the alkalimetals. It is also possible to use a mixture of solvent of the alkalimetal or a mixture of a solvent with a non-solvent inert with regard tothe alkali metal.

The extraction may be effected in the cold or in the hot from the solidor liquid amalgam. The solution of alkali metal is subsequentlyevaporated in order to separate the metal from the solvent. Theexhausted amalgam is recycled for concentration after having addedthereto fresh quantities of dilute amalgam coming from the electrolysis.

It goes without saying that the alkali metal dissolved in liquid ammoniaor another solvent may be used without prior separation for thepreparation of derivatives of this metal, such as peroxides or amides.On the other hand, it is interesting to note that the alkali metalobtained by evaporation of the solvent is in a very finely dispersedstate, in which it is extremely reactive and can therefore also beeconomically used for the preparation of various derivatives.

An interesting method of carrying the invention into effect consists inthe extraction of the alkali metal from the amalgam by the hydroxide ofthis alkali metal, or of another alkali metal, in the state of fusion.

It has been known that fused caustic soda dissolves sodium (Von Hevesy,Z. f. Electroch, August 1909, p. 529), but it has now been discoveredthat sodium, combined with mercury or with another metal in an amalgam,may be extracted by fused caustic soda.

According to the nature of the invention, the amalgam of liquid alkalimetal, at a concentration of the alkali metal exceeding by weight, isbrought into contact with the fused hydroxide of alkali metal in a tightreceptacle in order to extract part of the alkali metal contained in theamalgam, whereafter the exhausted amalgam is separated and returned intothe cycle for reconcentration while the alkali metal is separated fromthe hydroxide of alkali metal.

In the case of the extraction of sodium from its amalgam by causticsoda, this extraction may be effected at a temperature of above 325 C.,for instance between 325 and 600 C. Nevertheless, the temperature mayalso. be lowered below the normal melting point of caustic soda by theaddition of compounds such as NazCOa, KOH, NaBr, NaI, and the like.

The sodium dissolved in caustic soda may be separated onaccount of thefact that the solubility of sodium in caustic soda is inverted, thatmeans it decreases when the temperature increases. Thus, on heating thesolution, a layer of sodium floating on fused caustic soda separates.

' This separation may likewise be effected after cooling, by means of anauxiliary solvent.

In order to understand more easily the statement in the presentinvention, it has been assumed that the alkali metal dissolves in itsfused hydroxide. In reality, the matter in question is not so simple aphysical phenomenon and, under certain conditions, the dissolution isaccompanied by chemical reactions. It is known, for instance, that,above 450 C., and atmospheric pressure, the sodium reacts with causticsoda to form sodium oxide according to the reaction:

This reaction may be applied in a process in which the extraction ofsodium from an amalgam is combined with the production of sodium oxide.Nevertheless, when working under other conditions, particularly attemperatures, below 450 C. and/or under hydrogen pressure, it ispossible. to recuperate the. sodium in a non-combined state.

The assumed simplification of the. reaction mechanism of the hydroxideof alkali metal cannot therefore be harmful to the value of the process.

The following examples of the extraction of sodium from amalgam aregiven on the understanding that the invention is not limited to theseexamples, but that the process of the invention, as indicated above, isapplicable to the separation of any alkali metal from its amalgam.

These examples relate to continuous working methods of the process;however, the application may easily be effected in a discontinuousmanner by the successive extractions of the alkali metal from amalgam,the exhausted amalgam being treated in an extraction apparatus withfresh solvent until the concentration of the amalgam drops to the limitat which the extraction ceases to be economical.

Example 1 The diagram of the method can be seen from the accompanyingdrawing.

In a cell provided with a mobile mercury cathode, an aqueous solution ofsodium chloride is electrolysed to form an amalgam with 0.4% Na.

250 kg. of this hot liquid amalgam coming from 7 and mixed with 2.08 kg.of exhausted amalgam with 26% Na originating from extraction column 3,are introduced in a continuous manner into distillation column 1 withinone hour, where the amalgam is concentrated until an Na-content of 50%by weight is reached, the temperature not exceeding 550 C. From thiscolumn, 249 kg. of mercury vapors are separated within one hour, whichescape through 6, and which are condensed by utilising the condensationheat and the sensible heat by known methods. The cooled liquid mercuryis returned to the electrolysing cell. Furthermore, this column yields,at the same time, 3.08 kg. of concentrated amalgam with 50% Na which isdirected to heat exchanger 2 to be cooled to about 70 C.

The amalgam cooled in 2 is pumped into extraction column 3 where itmeets in counter-current liquid ammonia coming from 5. The sodiumextraction apparatus works under an ammonia pressure of 35 kg./cm. Theoutgoing solution leaves the apparatus at its upper part, and iscompressed to 62 kg./cm. in evaporator 4, Where 1 kg. of metallic sodiumper hour is separated.

The ammonia vapors liberated in the evaporation are expanded in anexpansion vessel (not shown) and condensed at 70 C. in condenser 5. Theliquid ammonia is reintroduced at the bottom of extraction column 3under pressure of 35 kg./cm.

The amount of liquid ammonia circulating by recycling for the extractionof 1 kg. of sodium per hour amounted to about 10 kg. per hour.

Example 2 2.00 kg. of amalgam with 0.5% of sodium produced byelectrolysis, are introduced into distillation column 1 Within one hoursimultaneously With 1.4 kg. of solid amalgam with 14.3% Na originatingfrom extractor column 3.

2.4 kg. of amalgam with 50% Na .are withdrawn from the bottom of thedistillation column and are cooled in a heat exchanger, then the liquidamalgam is dispersed in extraction column 3 wherein an ascending streamof liquid ammonia is circulating at 37 C. under atmospheric pressure.The finely dispersed solid amalgam drops to the bottom of the extractioncolumn while losing the sodium which dissolves in the liquid ammonia.The solution is heated to -20 C. in evaporator 4 soas. to deposit thesodium by the evaporation of These vapors are condensed atv 35 C. in 5and reintroduced into extraction column 3.

The quantity of liquid ammonia necessary for the extraction. whichcirculatesv by recycling in order to extract 1 kg. of sodium per hourwas 5 kg./hour approximately.

Example 3 1 kg. of previously fused and dried caustic soda is introducedinto a tight receptacle, then 1 kg. of fused sodium amalgam containing500 gr. of sodium is added. Two liquid layers are formed in thereceptacle, one layer of caustic soda and one layer of concentratedamalgam which floats on top. The whole is heated to a temperature of 480C. so as to extract the sodium from the amalgam by dissolving in causticsoda.

This extraction manifests itself by the inversion of the two liquidlayers. In effect, the amalgam becomes poorer in sodium, and itsspecific weight increases until it becomes superior to that of the fusedcaustic soda which becomes richer in sodium. An ascendant displacementof the caustic soda thus results which facilitates the exchange of thephase.

At the end of a certain time, the fused amalgam residue (750 gr.)contains only 333 gr. of sodium per kg. of amalgam.

It is withdrawn from the bottom of the receptacle and returned forconcentration. The caustic soda containing 250 gr. of sodium per kg. ofNaOH is then heated to 600 C. in a closed vessel.

A portion of the sodium, about 150 gr., separates because of thedecrease of the solubility due to the increase of the temperature andforms a liquid layer which floats on top. It is separated by decanting.The caustic soda still containing 100 gr. of sodium per kg. of NaOH isadvantageously utilised for a subsequent extraction.

Example 4 The caustic soda residue according to Example 3 with aconcentration of 100 gr. of sodium per kg. of NaOH is brought back to atemperature of 480 C., and 600 gr. of amalgam containing 500 gr. ofsodium per kg. of amalgam are added thereto.

After the extraction of part of the sodium, the exhausted amalgam (450gr.) with 333 gr. of sodium per kg. of amalgam is separated. The causticsoda phase having absorbed additional 150 gr. of sodium is reheated to600 C., and 150 gr. of sodium are separated therefrom, corresponding toa difference of the solubility between the two temperatures.

We claim:

1. A process for extracting alkali metals from their amalgams whichcomprises concentrating an alkali metal amalgam having a concentrationof less than by weight of the alkali metal by distilling mercurytherefrom to increase the concentration of said alkali metal to a valueof at least 10% by weight, bringing the concentrated amalgam thusobtained into contact with a solvent having a solvent action upon saidalkali metal but being non-reactive with the mercury and the amalgam,thereby dissolving a portion of the alkali metal from the amalgam,separating the amalgam from the solution of alkali metal in the solventby physical means, and recovering the alkali metal from the solvent.

2. A process for extracting alkali metals from their amalgams as definedin claim 1, wherein the alkali metal in the amalgam is brought to aconcentration of 40 to 60% by weight.

3. A process for extracting alkali metals from their amalgams as definedin claim 1, wherein the solvent is liquid ammonia.

4. A process for extracting alkali metals from their 5 amalgams asdefined in claim 1, wherein the treatment with the solvent is effectedat atmospheric pressure and at a temperature at which the amalgam issolid.

5. A process for extracting alkali metals from their amalgams as definedin claim 1, wherein the treatment 10 with the solvent is efiected atsub-atmospheric pressure and at a temperature at which the amalgam isliquid.

6. A process for extracting alkali metals from their amalgams as definedin claim 5, wherein the liquid amalgam is dispersed in the solvent andthe solvent is maintained at a temperature below the solidificationtemperature of the amalgam.

7. A process for extracting alkali metals from their amalgams as definedin claim 1, wherein the solvent is fused alkali metal hydroxide.

8. A process for extracting alkali metals from their amalgams as definedin claim 7, wherein the alkali metal is recovered from solution in thefused alkali metal hydroxide by heating the solution under hydrogenpressure at a temperature higher than the temperature at which theamalgam was brought into contact with the fused alkali metal hydroxide.

9. A process for extracting alkali metals from their amalgams as definedin claim 7, wherein the amalgam is brought into contact with the fusedalkali metal hydroxide under conditions which give rise to the formationof alkali metal oxide with the liberation of hydrogen.

10. A process for extracting alkali metals from their amalgams whichcomprises concentrating an alkali metal amalgam having a concentrationof less than 10% by weight of the alkali metal by distilling mercurytherefrom to increase the concentration of said alkali metal to a valueof at least 10% by weight, bringing the concentrated amalgam thusobtained into contact with a solvent selected from the group consistingof liquid ammonia,

ethylenediamine and methylamine, separating the amalgam from thesolution of alkali metal in the solvent by physical means and recoveringthe alkali metal from the solvent.

11. A process for extracting alkali metals from their amalgams asdefined in claim 1, wherein the amalgam separated from the solventsolution of the alkali metal is mixed with a fresh batch of diluteamalgam being charged to the concentration step.

References Cited in the file of this patent UNITED STATES PATENTS1,570,467 Ewan Jan. 19, 1926 2,124,564 Gilbert et a1 July 26, 19382,224,814 Gilbert Dec. 10, 1940 OTHER REFERENCES Johnson et al.:Chemical Reviews, vol. 8, April 1931, pages 273 to 301.

MacMullin: Chemical Engineering Progress, volume 46, No. 9 (September1950), pp. 447-448.

1. A PROCESS FOR EXTRACTING ALKALI METALS FROM THEIR AMALGAMS WHICHCOMPRISES CONCENTRATING AN ALKALI METAL AMALGAM HAVING A CONCENTRATIONOF LESS THAN 10% BY WEIGHT OF THE ALKALI METAL BY DISTILLING MERCURYTHEREFROM TO INCREASE THE CONCENTRATION OF SAID ALKALI METAL TO A VALUEOF AT LEAST 10% BY WEIGHT, BRINGING THE CONCENTRATED AMALGAM THUSOBTAINED INTO CONTACT WITH A SOLVENT HAVING A SOLVENT ACTION UPON SAIDALKALI METAL BUT BEING NOT-REACTIVE WITH THE MERCURY AND THE AMALGAM,THEREBY DISSOLVING A PORTION OF THE ALKALI METAL FROM THE AMALGAM,SEPARATING THE AMALGAM FROM THE SOLUTION OF ALKALI METAL IN THE SOLVENTBY PHYSICAL MEANS, AND RECOVERING THE ALKALI METAL FROM THE SOLVENT.